Light and EM spectrum

• We see things when light travels from a source and is reflected by an object into our eyes. The light transfers energy from the source to our eyes. Light is a type of electromagnetic wave.
• Visible light


  The frequencies of light that our eyes can detect
• Visible light


  • Lower frequencies appear more red
  • Higher frequencies appear more blue
• Some animals, such as birds, can also detect electromagnetic waves with frequencies that are higher than visible light. Electromagnetic waves with frequencies a little higher than visible light are called ultraviolet (UV).
• All electromagnetic waves are transverse waves. This means that the electromagnetic vibrations are at right angles to the direction in which the energy is being transferred by the wave. All electromagnetic waves travel at the same speed (3 x 10 m/s) in a vacuum. Like all waves, electromagnetic waves transfer energy from a source to an observer.
• Infrared (IR)


  Electromagnetic waves with frequencies slightly lower than visible light
• All objects emit energy by infrared radiation. The hotter the object the more energy it emits.
• Refraction


  The change in direction of a wave due to a change in its speed
• Light waves slow down when they go from air into water or glass. Different frequencies of light slow down by different amounts, so they are bent by different angles. This is why a prism can be used to split up visible light into the colours of the spectrum.
• The first person to investigate infrared radiation was the British astronomer William Herschel (1738-1822). He put dark, coloured filters on his telescope to help him observe the Sun safely. He noticed that different coloured filters heated up his telescope to different extents and he wondered whether the different colours of light contained different 'amounts of heat'.
• Light is refracted when it leaves the glass block, causing the cat to look much closer.
• Some animals can sense organs that can detect infrared radiation, such as pit vipers that have heat-sensing pits under their heads to detect warm-blooded prey.
• Visible light is part of a family of waves called electromagnetic waves. Our eyes can detect different colours in visible light. Scientists describe seven colours in the visible spectrum: red, orange, yellow, green, blue, indigo, violet.
• Electromagnetic spectrum


  The full range of electromagnetic waves, from the longest wavelength (lowest frequency) radio waves to the shortest wavelength (highest frequency) gamma rays
• Ultraviolet radiation has a higher frequency than visible light. Even higher frequencies and shorter wavelengths are present in X-rays and then gamma rays.
• Stars and other space objects can emit energy at all wavelengths. Astronomers use telescopes to study this radiation but they need to use different kinds of telescope to study different wavelengths.
• Most telescopes use curved mirrors to focus the electromagnetic radiation onto a central sensor. The type of material used for the mirror and the size of the telescope depend on the wavelength of the radiation being studied.
• Visible light is the part of the electromagnetic spectrum that our eyes detect. Light bulbs are designed to emit visible light, while cameras detect it and record images.
• Infrared radiation


  Can be used for communication at short ranges, such as between computers in the same room or from a TV to its remote control unit. The information sent along optical fibres is also sent using infrared radiation.
• Infrared radiation


  A grill or toaster transfers energy to food by infrared radiation. The food absorbs the radiation and heats up. Thermal images show the amount of infrared radiation given off by different objects.
• Infrared radiation


  Security systems often have sensors that can detect infrared radiation emitted by intruders. Some buildings are fitted with systems of infrared beams and detectors - someone walking through one of these beams breaks it and sets off the alarm.
• Microwaves


  Used for communications and satellite transmissions, including mobile phone signals. In a microwave oven, microwaves transfer energy to the food, heating it up from the inside.
• Radio waves


  Used for transmitting radio broadcasts and TV programmes as well as other communications. Some radio communications are sent via satellites, Controllers on the ground communicate with spacecraft using radio waves.
• Radio waves are produced by oscillations (variations in current and voltage) in electrical circuits. A metal rod or wire can be used as an aerial to receive radio waves. The radio waves are absorbed by the metal and cause oscillations in electric circuits connected to the aerial.
• Refraction


  The bending of the path of a wave due to a change in velocity
• Some frequencies of radio waves can be refracted by a layer in the atmosphere called the ionosphere. If radio waves reach the ionosphere at a suitable angle, they may be refracted enough to send them back towards the Earth. Microwaves are not refracted in the Earth's atmosphere.
• Ultraviolet radiation transfers more energy than visible light. It is absorbed by most of the same materials that absorb visible light, including our skin. The energy transferred can be used to disinfect water by killing microorganisms in it.
• Some materials absorb ultraviolet radiation and re-emit it as visible light. This is called fluorescence. Fluorescent materials are often used in security markings - they are only visible when ultraviolet light shines on them.
• Many low energy light bulbs are fluorescent lamps. A gas inside these lamps produces ultraviolet radiation when an electric current passes through it. A coating on the inside of the glass absorbs the ultraviolet and emits visible light.
• 1. rays


  Can pass through many materials that visible light cannot. For example, they can pass through muscles and fat easily but bone absorbs some X-rays. This means X-rays can be used in medicine to make images of the inside of the body. X-rays can also be used to examine the insides of metal objects and to inspect luggage in airport security scanners.
• Gamma rays


  Transfer a lot of energy, and can kill cells. For this reason, they are used to sterilise food and surgical instruments by killing potentially harmful microorganisms.
• Gamma rays


  Used to kill cancer cells in radiotherapy. They can also be used to detect cancer. A chemical that emits gamma rays is injected into the blood. The chemical is designed to collect inside cancer cells. A scanner outside the body then locates the cancer by finding the source of the gamma rays. Gamma rays can pass through all the materials in the body.
• All waves transfer energy. A certain microwave frequency can heat water and this frequency is used in microwave ovens. This heating could be dangerous to people because our bodies are mostly water and so the microwaves could heat cells from the inside. Mobile phones use different microwave frequencies. Current scientific evidence tells us that, in normal use, mobile phone signals are not a health risk.
• Infrared radiation is used in grills and toasters to cook food. Our skin absorbs infrared, which we feel as heat. Too much infrared radiation can damage or destroy cells, causing burns to the skin.
• Radiation


  Higher-frequency waves transfer more energy than low-frequency waves and so are potentially more dangerous. Sunlight contains high frequency ultraviolet radiation, which carries more energy than visible radiation. The energy transferred by ultraviolet radiation to our cells can cause sunburn and damage DNA. Too much exposure to ultraviolet radiation can lead to skin cancer.
• The ultraviolet radiation in sunlight can also damage our eyes. Skiers and mountaineers can suffer temporary 'snow blindness' because so much ultraviolet radiation is reflected from snow. We can protect our eyes using sunglasses.
• 1. rays and gamma rays are higher frequency than ultraviolet radiation and so transfer more energy. They also can penetrate the body. Excessive exposure to X-rays or gamma rays may cause mutations in DNA that can kill cells or cause cancer.
• Electromagnetic radiation is produced by changes in the electrons or the nuclei in atoms. For example, when materials are heated, changes in the way the electrons are arranged can produce infrared radiation or visible light. Changes in the nuclei of atoms can produce gamma radiation.
• Radiation can also cause changes in atoms, such as causing atoms to lose electrons to become ions.